1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine equipped with exhaust gas recirculation (EGR) system.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air/fuel ratio of the engine is controlled around a stoichiometric value (14.7:1 for example) in accordance with the composition of exhaust gas and the operating conditions of the engine in order to provide an optimum operation of the three-way catalytic converter. The so called air intake side secondary air supply system is an example of the air/fuel ratio control system of this type which has a secondary air passage leading to a portion of an intake air passage downstream of the throttle valve and in which the amount of the secondary air passing therethrough is controlled by varying the sectional area of the secondary air passage.
The secondary air supply system is often provided with an air control valve which is disposed in the secondary air supply passage for varying the sectional area thereof and the opening degree of the air control valve is controlled according to the magnitude of the pressure applied to a pressure chamber of the air control valve. In this case, the pressure chamber is supplied with a control pressure which varies with the operating conditions of the engine. Further, an actual air/fuel ratio is detected from an oxygen concentration of the exhaust gas, and the secondary air is supplied, via the air control valve, to a downstream part from the throttle valve, for example by controlling the control pressure, when the detected air/fuel ratio is rich and the supply of the secondary air is stopped or the amount of the secondary air is gradually reduced when the detected air/fuel ratio is lean.
In internal combustion engines, it is also necessary to decrease the combustion temperature in the cylinders to prevent the formation of NOx (Nitrogen Oxides) which is one of noxious components contained in the exhaust gas of the engine. Therefore, some internal combustion engines are provided with an exhaust gas recirculation (EGR) system for reducing the combustion temperature, which recirculates a part of exhaust gas into the intake air passage to slow down the combustion process to an extent which does not affect the engine performance. The exhaust gas recirculation system generally includes an EGR passage connecting the exhaust passage and the intake air passage, and a vacuum operated EGR flow control valve disposed in the EGR passage for varying the sectional area of the EGR passage in accordance with the level of the pressure applied to a pressure chamber thereof. The pressure chamber of the EGR flow control valve is generally applied with a control pressure which varies with the operating conditions of the engine.
If an internal combustion engine equipped with the EGR system is provided with the air intake side secondary air supply system, it is necessary to vary the opening degree of the air control valve and the EGR flow control valve according to various operating conditions of the engine. Therefore it is desirable to supply a control pressure from a single source of the control pressure, which varies with the operating conditions of the engine, an amount of the intake air for example, to pressure chambers of the air control valve and the EGR control valve to simplify the total construction of the peripheral systems of the internal combustion engine and to reduce production costs.
On the other hand, in the air intake side secondary air supply system, an open/close valve may be disposed in an air intake side secondary air passage so as to stop the supply of the air intake side secondary air in accordance with the detected result of the air/fuel ratio which is determined from the above mentioned exhaust gas composition. In this case, the response of the open/close valve against a secondary air supply command is relatively good if the open/close valve is disposed in the air intake side secondary air supply passage downstream of the air control valve. However, if the difference of the atmospheric air pressure and the vacuum of the downstream of the throttle valve becomes large when the vacuum of the downstream of the throttle valve is applied to the valve element of the air control valve such as a needle valve via the air intake side secondary air passage and the open/close valve, the valve element of the air control valve tends to be sucked to the direction to close the valve or moved to the direction to close the valve. This motion of the valve element will in turn cause a motion of the diaphragm which forms a part of the pressure chamber of the air control valve, and causes the expansion and the contraction of the volume of the pressure chamber and the pulsation of the pressure in the pressure chamber. However, when the air control valve and the EGR control valve are supplied with the pressure from the same source of the control pressure as mentioned above, and the pressure chamber of the air control valve is communicated with the pressure chamber of the EGR control valve via the pressure supply passage, the problem is that the pulsation of the pressure in the pressure chamber of the air control valve in turn causes the pulsation of the pressure in the pressure chamber of the EGR valve due to the communication through the pressure supply passage. This pulsation may become a cause of various adverse effects to the operation of the EGR system.